Black carbon scavenging by low-level Arctic clouds

Black carbon (BC) from anthropogenic and natural sources has a pronounced climatic effect on the polar environment. The interaction of BC with low-level Arctic clouds, important for understanding BC deposition from the atmosphere, is studied using the first long-term observational data set of equivalent black carbon (eBC) inside and outside of clouds observed at Zeppelin Observatory, Svalbard. We show that the measured cloud residual eBC concentrations have a clear seasonal cycle with a maximum in early spring, due to the Arctic haze phenomenon, followed by cleaner summer months with very low concentrations. The scavenged fraction of eBC was positively correlated with the cloud water content and showed lower scavenged fractions at low temperatures, which may be due to mixed-phase cloud processes. A trajectory analysis revealed potential sources of eBC and the need to ensure that aerosol-cloud measurements are collocated, given the differences in air mass origin of cloudy and non-cloudy periods.

(b) Dependence of eBC on surface type for non-cloudy periods (visibility>5 km).Here, each endpoint is denoted as being over land (1) or ocean (-1), and the average of all endpoints within the mixed-layer for each ensemble is then assigned to an observation.Hence, an ensemble which traverses only over the ocean is given the value -1.The fill color of the box plots denotes the values of the x-axis.The centre line of the boxes represents the median, while the extent of the boxes show the interquartile range.The whisker show the range of data (defined as 1.5 times the interquartile range from the nearest quartile).N in the panel title denotes the total number of available data points.
Figure S1.Surface residence time for cloudy and non-cloudy periods for entire 4-year data set.(a) Surface residence time for cloudy periods with GCVI sampling (visibility<1 km).(b) Surface residence time for non-cloudy periods (visibility>5 km).Grid cells in which no back trajectories traversed over are given black edges.The surface residence times were normalised by dividing by the maximum count found in the grid cells (not including the grid cell in which ZEP is located).
Figure S2.Marine-continental influence.(a) Dependence of eBC on surface type for cloudy periods with GCVI sampling (visibility<1 km).
Figure S3.Percentage spent over the ocean of cloudy and cloud-free periods spent over each surface type (i.e.land or ocean) and within the mixed-layer (ML) is normalized by the total spent time within the ML.The average time spent over the ocean for cloudy periods equates to 86.85%, and 80.56% for cloud-free periods.

Figure S4 .
Figure S4.The annual cycle of the scavenged fraction of eBC.Scavenged fraction of eBC as a box plot for all data, and for vertical wind values below 1 ms −1 and 0.5 ms −1 , respectively.Shown are monthly mean and median values as solid and dashed line, respectively.The centre line of the boxes represents the median, while the extent of the boxes show the interquartile range.The whisker show the range of data (defined as 1.5 times the interquartile range from the nearest quartile).All box plots contain hourly mean values.The numbers in blue, light blue and grey give the number of hourly value contained in each month for each data class (see legend).

Figure S5 .
Figure S5.The scavenged fraction of eBC binned by cloud water content for two different temperature regimes.(a) Ambient temperature above -5 • C and (b) for ambient temperature below -5 • C. The number of points per box (15 in panel a and 10 in panel b) is given above each subplot.The centre line of the boxes represents the median, while the extent of the boxes show the interquartile range.The whisker show the range of data (defined as 1.5 times the interquartile range from the nearest quartile).The shading of the color of the box plots denotes the values of the x-axis.The exponential fits in panel a are shown for the 1-h mean values (orange dashed curve) and for the binned median values (red curve), respectively together with their corresponding 95 % confidence intervals (shaded area).The corresponding fit coefficients are given in the legend together with their 95 % confidence intervals.No reasonable exponential fit was possible for panel b.

Figure S6 .
Figure S6.eBC concentrations binned by cloud water content (CWC) and annual cycle of CWC and visibility.(a) eBC within cloud residuals vs. CWC.(b) eBC of whole-air (during cloudy periods) vs. CWC.(c) Monthly values of CWC.(d) Corresponding monthly values of visibility.The centre line of the boxes represents the median, while the extent of the boxes show the interquartile range.The whisker show the range of data (defined as 1.5 times the interquartile range from the nearest quartile).The number of 1-h mean values in each box is given above the panel (a-b) or as grey number within the panel (c-d).The data in panel (a) and (b) is shown for all ambient temperatures.

Figure S7 .Figure S8 .
Figure S7.Normalised histogram of the occurrence of cloudy and cloud-free periods versus wind direction.Two wind directions prevail at Zeppelin Observatory: north-north-westerly winds which tend to bring more clouds, while south-south-easterly winds tend to bring less cloudy air.The difference between cloudy periods (visibility <1 km) with and without GCVI operation are shown with full and empty bars in blue.

Figure S10 .
Figure S10.Concentration difference in eBC after the CVI was switched on (orange lines) or off (black lines).The switching of the GCVI caused fluctuations in the eBC readings due to humidity effects within the MAAP instruments.Values within 15 min after the switching of the GCVI were disregarded.

Figure S11 .
Figure S11.Available observations.Hours of observations during non-cloud periods (visibility>5 km, red dashed curve) and during cloudy periods when the GCVI was in operation (visibility<1 km).Shown are periods when both MAAP instruments measured eBC (blue solid line) and when corresponding auxiliary data was available for ambient temperature (blue dashed line), cloud water content (blue dashed dotted line) and updraft (blue dotted line).